390 / 2019-02-22 03:34:51

Cytokinin induced LBD genes regulate root secondary growth in Arabidopsis thaliana

cytokinin, secondary growth

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Vascular cambium produces secondary wood (inwards) and secondary phloem (outwards) in the radial dimension, which leads to increased thickness of plant. Cytokinins were shown to regulate cambium activity and secondary growth. When 4 genes encoding cytokinin biosynthetic isopentenyltransferases (IPT1;3;5;7) were disrupted, the plant was unable to form cambium and showed reduced thickening of the root and stem (Matsumoto-Kitano et al., 2008). However, molecular basis of how cytokinin regulate secondary growth is largely unknown. Here, through a comparative analysis with time-series microarray profiling data of cytokinin-treated atipt1;3;5;7 roots, over 2000 genes were identified. Network analysis of cytokinin regulated transcriptional factor showed several members of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family expressed in root vascular region. Their expression increase as root thickens, indicating they may have an important role in secondary growth. Induced overexpression of these LBD genes in Arabidopsis root showed more cell proliferation and cell expansion in root vasculature, thus resulting in increased secondary growth. Even though only weak phenotypes were observed in single knock-out mutants, created by CRISPR-CAS9 system, vascular pattern became defective and root thickening was reduced when combining these lbd mutants. These results reveal these LBD genes act as novel factors downstream of cytokinin signaling pathway during root secondary growth in Arabidopsis.